Phase-contrast imaging with X-ray beams is based on a grating-based interferometer, which utilizes the Talbot effect for imaging. To this end, two gratings are positioned parallel to one another, perpendicular to the X-ray beam. The phase grating g1 consists of lines which cause a phase shift of π (or π/2) (π=3.14159 . . . ) and an X-ray absorption that can be neglected. It acts as a beam splitter and divides the X-ray beams into the +1 and −1 diffractive order. The diffracted beams interfere according to the (fractional) Talbot effect and an interference pattern is created, which is periodic in the directions perpendicular to the grating lines. The interference image has a maximum modulation at the fractional Talbot distances dm:
For parallel beam geometry, dm is
            d      m        =          m      ⁢                        p          1          2                          8          ⁢                                          ⁢          λ                      ,where:    λ (lambda)=wavelength,    p1=grating period of g1 (phase grating),    m=odd integer=order of the fractional Talbot distance.
For the conventional cone-beam geometry, dm changes by the factor
      l          l      -              d        m              ,where 1 is the distance of the phase grating from the X-ray source.
In order to determine the position of the interference pattern, it is scanned by the analyzer grating g2, which has strongly absorbing slats with the period p2.
For the cone-beam geometry, p2 emerges from
            p      2        =                  l                  l          -                      d            M                              ⁢                        p          1                A              ,with A=1 for a phase shift of π/2 and A=2 for a phase shift of π (π=3.14159 . . . ). As a result, the ratio of the periods p1 and p2 depends on the wavelength, on the selected fractional Talbot distance and on the distance 1 of the phase grating from the X-ray source.
Since deviations from the ideal period ratio lead to a deterioration of the image quality, a grating set fitted to the period must be selected for each experiment. Since the production of the gratings using micro-technical methods is very complicated, this is very cost intensive. Moreover, the user does not have the option of performing measurements at different Talbot distances using a predetermined grating set and thus matching the measurement set-up to different sensitivities of the objects to be measured. A further problem consists in the fact that, depending on the production method, the period can only be maintained to an accuracy of a few nanometers. By way of example, the positional accuracy of an electron beam writer, which is usually used for creating the primary grating structure, is specified to a few nanometers. In this respect, variations in the period length between phase grating and analyzer grating of a few nanometers are to be expected, particularly in the case of gratings for which an electron beam lithography is necessary in each case.
A person skilled in the art is sufficiently well aware of more detailed information in respect of the described effects and apparatuses; therefore it does not have to be described in any more detail here. US 2007/0183579 A1 and US 2009/0092227 A1 have disclosed to rotate phase gratings about the optical axis in accordance with the direction of incidence of the beams, in order thereby to counteract bothersome moiré patterns.
DE 100 25 694 A1 relates to Littrow configurations when using UV beams (weak UV beams).
US 2009/0207416 A1 describes instruments based exclusively on light (optionally UV and IR), but not on X-ray beams.
In DE 10 2008 048 668 A1, an attempt is made to sidestep the effect of the spreading of the X-ray beam by arranging a plurality of gratings, i.e. what should be achieved by arranging a plurality of gratings next to one another is that the X-ray beams pass straight through the grating structure because these grating structures are perpendicular to the respectively incident X-ray beams.
Object
The object of the present invention is to avoid the disadvantages of the prior art.
In particular, the image quality, for example the phase contrast and focus, should be improved by simple means; interferometers should be usable in a more variable fashion such that there is no need for each experiment to have its own phase grating, resulting in economical advantages.
Solution
This object is achieved by special gratings or grating arrangements for the phase gratings of interferometers, and by corresponding interferometers and methods, and also by uses, in which the grating structure is inclined in relation to the irradiation direction or the optical axis, corresponding to a rotation of the grating about the grating central axis which runs parallel to the grating ribs or a rotation of the individual grating ribs about their respective longitudinal axis which is arranged parallel to the grating central axis.
Terminology
Within the scope of the present invention, the terms “interferometer” and “grating interferometer” relate to instruments which work using the Talbot effect.